Brad Plumer wonders if the move is simply political:
According to the EIA, gas prices are expected to go up quite a bit this summer (probably shooting north of $3/gallon), and the administration may want to step out ahead of the inevitable teeth-gnashing and garment-rending over the issue. So this could be more about the midterms than rounding up votes in the Senate. Though, granted, this drilling announcement won't affect summer gas prices in the slightest.
I don't recall Obama ever railing against this in the campaign, although I might be wrong. I see it not as a mid-term tactic but, once again, as a strategic move to show he is open to ideas from his opponents, while his opponents are rigidly and ideologically opposed to anything he might suggest. It slowly seeps in - among Independents and even among many traditional Republicans - that he is the reasonable guy in the room.
Goodbye to all that, remember? And you do it gradually, undramatically, constantly. Meep, meep.